1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display apparatus.
2. Description of the Related Art
The research and development of organic EL display apparatuses each using organic EL devices have been vigorously performed in recent years.
Each of the organic EL display apparatuses has multiple red organic EL devices, multiple green organic EL devices, and multiple blue organic EL devices, and the devices each independently act as a pixel to perform emission and non-emission. As a result, each of the apparatuses can display a full-color image.
Each of the organic EL devices has a pair of electrodes and an emission layer formed of an organic compound interposed between the electrodes. The emission layer has a fluorescent light emitting material or phosphorescent light emitting material as itself or as, for example, a guest material having a small weight ratio.
Each of the organic EL devices for the respective colors has been developed so as to be capable of being driven at a low voltage. At the time of the development, a layer constitution in each of the organic EL devices for the respective colors is designed so that: a difference of driving voltage among the organic EL devices of the respective colors may be prevented from becoming large; and a device for any emission color may be capable of being driven at a low voltage.
While the development of a fluorescent light emitting material or phosphorescent light emitting material has been performed, Japanese Patent Application Laid-Open No. 2004-241374 describes that a delayed fluorescent material is used in an organic EL device.
A fluorescent light emitting material is theoretically hard to show an internal quantum efficiency of 100%. On the other hand, a phosphorescent light emitting material can theoretically show an internal quantum efficiency of 100%.
However, in the case of an organic EL device having an emission layer in which the phosphorescent light emitting material is incorporated as a guest material into a host material, the band gap of the host material cannot help being expanded as compared to that in the case where the emission layer has the fluorescent light emitting material that emits light of the same color as that of light emitted from the phosphorescent light emitting material.
When light of a certain color is to be emitted, the lowest excited triplet state T1 of the phosphorescent light emitting material must be at an energy level corresponding to the color. The lowest excited singlet state S1 of the phosphorescent light emitting material is higher than the T1. In addition, the T1 of the host material having the phosphorescent light emitting material is higher than the T1 of the phosphorescent light emitting material, and the S1 of the host material is higher than the T1 of the host material.
In the case of the fluorescent light emitting material that emits light of the same color as that described above, an excited state corresponding to the color is not the T1 but the S1. That is, when the fluorescent light emitting material and the phosphorescent light emitting material emit light of the same color, the S1 of the fluorescent light emitting material is lower than the S1 of the phosphorescent light emitting material. Accordingly, when the emission layer has the phosphorescent light emitting material as a guest material, the S1 of the host material cannot help being made higher than that in the case where the emission layer has the fluorescent light emitting material. As a result, the band gap of the host material cannot help being expanded.
The following procedure plays an important role in moving a carrier (an electron or a hole) from a layer adjacent to the emission layer to the emission layer: an energy barrier between the emission layer and the adjacent layer is not expanded. As the band gap of the host material expands, highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) of the host material starts to differ from the HOMO or LUMO of the adjacent layer. As a result, the barrier becomes high.
In that case, the following procedure is needed: an optimum material for the adjacent layer is selected, or a novel compound is created to serve as a material for the layer. In addition, in that case, whether the extent to which the carrier is injected from an electrode is large or small must be taken into consideration. As a result, the layer constitution of an organic EL device must be designed again from scratch.
By the way, Japanese Patent Application Laid-Open No. 2004-241374 describes the delayed fluorescent material. The delayed fluorescent material showed each of a strong delayed fluorescent spectrum and a strong phosphorescent spectrum in the range of 520 nm to 750 nm, and an emission wavelength actually illustrated in the figure is formed of a peak having a maximum emission wavelength in excess of 550 nm and a peak having a maximum emission wavelength in excess of 600 nm. That is, the delayed fluorescent material is not a light emitting material that emits light of a primary color such as a green or blue color in view of its color purity.